Introduction to Mendelian Genetics. Some History For thousands of years farmers and herders have been selectively breeding their plants and animals to.

Introduction to Mendelian Genetics

Some History

• For thousands of years farmers and herders have been selectively breeding their plants and animals to produce more useful hybrids• It was somewhat of a hit or miss process since the actual mechanisms governing inheritance were unknown• Knowledge of these genetic mechanisms finally came as a result of careful laboratory breeding experiments carried out over the last 150 years• Started with the studies of an Austrian monk = Gregor Mendel

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Who was Gregor Mendel?• He is known as the

“FATHER OF GENETICS” • He discovered how traits were

inherited – the 1st scientist to obtain successful results from inheritance studies due to his methodology

• Lived: 1822 – 1884• His work was “unimportant”

until early 1900’sGENETICS – study of heredity

HEREDITY – the passing of traits from parents to offspring

Mendel’s Home & Garden

Mendel’s Peas• Mendel did his study on 29,000 pea plants over 14 years

• Pea plants have many easily observed traits (tall/short, purple flowers/white flowers)=phenotypes

• Pea plants can beself-fertilized or cross-fertilized (cross pollinated)

Cross Pollination

(1) The pollen contains the male gamete and can be “picked up” by a fine brush

(2) The carpel is the female reproductive structure in a flower and contains the ovary where fertilization of the female gamete occurs

(3) Fertilization occurs after pollen has landed (or placed) upon the carpel

Genetic Terminology

1.True-Breeding – (PURE BRED) these organisms ALWAYS create offspring that look like themselves (same phenotypes or traits)

2.Hybrids – offspring from different true-breeding organisms

Tall purebred x Short prurebred = Hybrid

Ideas about traits during Mendel’s time

Most scientists thought that traits blended in offspring to produce “middle ground” offspring

Parent ParentOffspring

Mendel’s Observations & Ideas

While observing many generations of pea plants, Mendel found that sometimes traits seemed to disappear for a generation and then reappear in later generations.

He determined:

Some factor MUST be passed from generation to generation for this to occur.

(We now know these as GENES)

Those traits that disappeared were recessive to other traits that were dominant.

(GENES can be in different forms = ALLELES).

Genes and Alleles

Genes – factors that determine your traits

Mendel determined that each trait is controlled by two factors (alleles)

One from male, one from female – they can be different)

Mendel’s Experiments

Parents = P First generation offspring = F1

Second generation offspring = F2

Observable trait = phenotype Genetic makeup = genotypeGenotype is represented by capital and lower case letters

Pure Bred Hybrid Hybrid

Explaining the Cross Results

When a parent makes sperm or eggs, their genes separate randomly        (PRINCIPLE OF SEGREGATION)

The GAMETES (egg or sperm) contain either a T allele (tall) or a t allele (short)

1. Principle of Dominance and RecessivenessOne allele in a pair may mask the effect of the other

2. Principle of SegregationThe two alleles for a characteristic separate during the

formation of eggs and sperm

3. Principle of Independent AssortmentThe alleles for different characteristics are distributed to

reproductive cells independently

Three important conclusions to Mendel’s research